Light-emitting diodes (LEDs) belong to a class of the most efficient light sources among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL) for backlight, and halogen lamps. As one configuration for white LED, a white LED device (LED illuminator) constructed by combining a blue light-emitting diode (blue LED) with a phosphor capable of emitting light of longer wavelength, for example, yellow or green light upon blue light excitation is implemented on a commercial basis.
The mainstream of the white LED structure is a system in which a phosphor in admixture with resin or glass is placed on or near a blue LED so that the phosphor layer substantially integrated with the blue LED may convert the wavelength of part or all of blue light to produce pseudo-white light, to be called white LED element system. Also some light-emitting devices are based on a system in which a phosphor is spaced apart from a blue LED by a distance of several millimeters to several tens of millimeters so that the phosphor may cause wavelength conversion to part or all of blue light.
Particularly when the phosphor tends to degrade its properties by the heat generated by LED, the far distance of phosphor from the LED is effective for improving the efficiency of light-emitting device and suppressing the variation of color tone. A phosphor-containing wavelength conversion member to be spaced apart from an LED light source is known as remote phosphor plate, and such a light emitting system is known as “remote phosphor technology.” Recently is active efforts are made on the light emitting system of remote phosphor technology because an improvement in overall color variation and other improvements are advantageous when the system is used for illumination.
The light-emitting device of remote phosphor technology is generally constructed, for example, by placing a wavelength conversion member, which is made of resin or glass having yellow light-emitting phosphor particles, green phosphor particles or red phosphor particles dispersed therein, forward of a blue LED as the remote phosphor, to provide a light emitting device adapted to produce white light. Examples of the phosphor used as the remote phosphor include Y5Al5O12:Ce, (Y,Gd) (Al, Ga)5O12:Ce, (Y,Gd)3Al5O12:Ce, Tb3Al5O12:Ce, (Sr, Ca, Ba)2SiO4:Eu, and β-SiAlON:Eu. Sometimes, CaAlSiN3:Eu2+, Sr—CaAlSiN3:Eu2+ and the like are used as the red phosphor for improving the color rendering of illuminators.
Of the aforementioned phosphors suited for LED illuminators, many examples of the green or yellow light-emitting phosphor are known, but known examples of the red light-emitting phosphor are not so many, such as nitride phosphors CaAlSiN3:Eu (CASN) and (Sr, Ca)AlSiN3:Eu (S-CASN) and oxynitride phosphors called □-SiAlON phosphor. Since these red light-emitting phosphors must generally be synthesized at high temperature and high pressure, a special equipment resistant to high temperature and high pressure is necessary for their mass-scale synthesis.
In the past, the inventors studied in JP-A 2012-224536 (Patent Document 1) a method for the synthesis of a red phosphor having the formula (1):A2(M1-xMnx)F6  (1)wherein M is one or more tetravalent elements selected from among Si, Ti, Zr, Hf, Ge, and Sn, A is one or more alkali metals selected from among Li, Na, K, Rb, and Cs and contains at least Na and/or K, and x is a number of 0.001 to 0.3, the red phosphor being promising as LED-compatible phosphor. With the synthesis method the inventors developed, the synthesis of the phosphor can be carried out at a low temperature of up to 100° C. and atmospheric pressure, and a phosphor having a satisfactory particle size and quantum efficiency is obtained. The phosphor, however, is still insufficient in durability at high humidity.